Studies are in progress to evaluate the structural and physiological effects of hyperosmotic media on cardiac muscle. Spontaneously beating atria exhibit increases in passive tension after 1 hour in hyperosmotic media. We are studying the ultrastrutural changes associated with this passive tension change which appears in many respects similar to contracture caused by high Ca 2 ion or hypoxia. In addition, we are evaluating the effects on atrial tension of other agents known to cause contracture in cardiac muscle for comparison with the hyperosmotic induced contracture. Electrophysiological studies are planned to determine the effect of hyperosmolar media on pacemaker potentials in the sinoatrial node.